Showing papers by "Dragan Maksimovic published in 2014"

Accounting for Lithium-Ion Battery Degradation in Electric Vehicle Charging Optimization

Abstract: This paper presents a method for minimizing the cost of vehicle battery charging given variable electricity costs while also accounting for estimated costs of battery degradation using a simplified lithium-ion battery lifetime model. The simple battery lifetime model, also developed and presented here, estimates both energy capacity fade and power fade and includes effects due to temperature, state of charge profile, and daily depth of discharge. This model has been validated by comparison with a detailed model developed at National Renewable Energy Laboratory, which in turn has been validated through comparison with experimental data. The simple model runs quickly, allowing for iterative numerical minimization of charge cost, implemented on the charger controller. Resulting electric vehicle (EV) charge profiles show a compromise among four trends: 1) charging during low-electricity cost intervals; 2) charging slowly; 3) charging toward the end of the available charge time; and 4) suppression of vehicle-to-grid power exportation. Simulations based on experimental Prius plug-in hybrid EV usage data predict that batteries charged using optimized charging last significantly longer than those charged using typical charging methods, potentially allowing smaller batteries to meet vehicle lifetime requirements. These trends are shown to hold across a wide range of battery sizes and hence are applicable to both EVs and plug-in hybrid EVs.

High-Frequency PWM Buck Converters Using GaN-on-SiC HEMTs

Abstract: GaN high electron mobility transistors (HEMTs) are well suited for high-frequency operation due to their lower on resistance and device capacitance compared with traditional silicon devices. When grown on silicon carbide, GaN HEMTs can also achieve very high power density due to the enhanced power handling capabilities of the substrate. As a result, GaN-on-SiC HEMTs are increasingly popular in radio-frequency power amplifiers, and applications as switches in high-frequency power electronics are of high interest. This paper explores the use of GaN-on-SiC HEMTs in conventional pulse-width modulated switched-mode power converters targeting switching frequencies in the tens of megahertz range. Device sizing and efficiency limits of this technology are analyzed, and design principles and guidelines are given to exploit the capabilities of the devices. The results are presented for discrete-device and integrated implementations of a synchronous Buck converter, providing more than 10-W output power supplied from up to 40 V with efficiencies greater than 95% when operated at 10 MHz, and greater than 90% at switching frequencies up to 40 MHz. As a practical application of this technology, the converter is used to accurately track a 3-MHz bandwidth communication envelope signal with 92% efficiency.

Active balancing system for electric vehicles with incorporated low voltage bus

Abstract: Electric-drive vehicles, including hybrid (HEV), plug-in hybrid (PHEV) and electric vehicles (EV), require a high-voltage (HV) battery pack for propulsion, and a low-voltage (LV) dc bus for auxiliary loads. This paper presents an architecture that uses modular dc-dc bypass converters to perform active battery cell balancing and to supply current to auxiliary loads, eliminating the need for a separate HV-to-LV high step-down dc-dc converter. The modular architecture, which achieves continuous balancing of all cells, can be used with an arbitrary number of cells in series, requires no control communications between converters, and naturally shares the auxiliary load current according to the relative state-of-charge (SOC) and capacities of the battery cells. Design and control details are provided for low-voltage, low-power dual active bridge (DAB) power converters serving as bypass converter modules. Experimental results are presented for a system consisting of two series 3.6 Ah NMC battery cells and two DAB bypass converters, with combined outputs rated to supply a 12 V, 35 W auxiliary load.

Control of Submodule Integrated Converters in the Isolated-Port Differential Power-Processing Photovoltaic Architecture

Abstract: Recently, a variety of differential power-processing (DPP) architectures have been shown to improve the efficiency of photovoltaic (PV) systems. This paper proposes a simple control strategy for the isolated-port DPP architecture, and provides a comprehensive stability analysis for this system. The proposed controller drives the duty-cycle of the differential submodule integrated converters (subMICs) in proportion to a voltage difference between the submodule and the isolated-port. This method requires no additional sensing, complex processing, or communication between subMICs, and is therefore well suited for low-cost integrated hardware solutions. Stability of the resulting high-order nonlinear system is analyzed both in the time and frequency domains. A decoupled model is developed that reduces the high-order system dynamics to a 1-D control loop, which allows stable, well-behaved responses using a proportional or a lag compensator. Experimental results for a 72-cell PV module with three subMICs verify static and dynamic operation, and show that overall PV module efficiency exceeds 99% with no shading, and is higher than 96% under significant (50%) shading.

Performance of Mismatched PV Systems With Submodule Integrated Converters

Abstract: Mismatch power losses in photovoltaic (PV) systems can be reduced by the use of distributed power electronics at the module or submodule level. This paper presents an experimentally validated numerical model that can be used to predict power production with distributed maximum power point tracking (DMPPT) down to the cell level. The model allows the investigations of different DMPPT architectures, as well as the impact of conversion efficiencies and power constraints. Results are presented for annual simulations of three representative partial shading scenarios and two scenarios where mismatches are due to aging over a period of 25 years. It is shown that DMPPT solutions that are based on submodule integrated converters offer 6.9-11.1% improvements in annual energy yield relative to a baseline centralized MPPT scenario.

Modular Approach for Continuous Cell-Level Balancing to Improve Performance of Large Battery Packs

Abstract: Energy storage systems require battery cell balancing circuits to avoid divergence of cell state of charge (SOC). A modular approach based on distributed continuous cell-level control is presented that extends the balancing function to higher level pack performance objectives such as improving power capability and increasing pack lifetime. This is achieved by adding DC-DC converters in parallel with cells and using state estimation and control to autonomously bias individual cell SOC and SOC range, forcing healthier cells to be cycled deeper than weaker cells. The result is a pack with improved degradation characteristics and extended lifetime. The modular architecture and control concepts are developed and hardware results are demonstrated for a 91.2 Wh battery pack consisting of four series li-ion battery cells and four dual active bridge (DAB) bypass DC-DC converters.

100 MHz, 20 V, 90% efficient synchronous buck converter with integrated gate driver

Abstract: This paper describes a synchronous buck converter based on a GaN-on-SiC integrated circuit, which includes a halfbridge power stage, as well as a modified active pull-up gate driver stage. The integrated modified active pull-up driver takes advantage of depletion-mode device characteristics to achieve fast switching with low power consumption. Design principles and results are presented for a synchronous buck converter prototype operating at 100 MHz switching frequency, delivering up to 7 W from 20 V input voltage. Measured power-stage efficiency peaks above 91%, and remains above 85% over a wide range of operating conditions. Experimental results show that the converter has the ability to accurately track a 20 MHz bandwidth LTE envelope signal with 83.7% efficiency.

Zero Voltage Switching Technique for Bidirectional DC/DC Converters

Abstract: This paper proposes a zero voltage switching (ZVS) technique for bidirectional dc/dc converters. The dc/dc unit considered consists of two distinct bidirectional dc/dc cells paralleled at both input and output and whose two input bridges are coupled by means of passive inductive branches. A multiangle phase-shift modulation method is proposed which simultaneously achieves bidirectional power control, power sharing, and ZVS of all the electronic devices over the full power range without the need for auxiliary switches. Simulation and experimental results are reported for a 2.4 kW dc/dc unit consisting of two paralleled 1.2 kW bidirectional dual-bridge series resonant converter cells.

High-frequency integrated gate drivers for half-bridge GaN power stage

Abstract: This paper describes three driver options for integrated half-bridge power stage using depletion-mode GaN-on-SiC 0.15μm RF process: an active pull-up driver, a bootstrapped driver, and a modified active pull-up driver. The approaches are evaluated and compared in 5 W, 20 V synchronous Buck converter prototypes operating at 100 MHz switching frequency over a wide range of operating points. Measured efficiency peaks above 91% for the designs using the bootstrap and the modified active pull-up integrated drivers.

A 98.7% efficient composite converter architecture with application-tailored efficiency characteristic

Abstract: A dc–dc boost composite converter architecture is introduced that can lead to optimized efficiency over a range of operating points dictated by the application requirements. The composite converter system employs dissimilar modules to minimize the ac power losses in the indirect power conversion paths. It is composed of three converter modules: buck converter, boost converter, and a dual active bridge converter that operates as a dc transformer (DCX). Each module processes partial power, with reduced voltage rating. With the same semiconductor area and same magnetics volume, substantial efficiency improvements and reductions in capacitor size are achieved relative to a conventional boost architecture. It is possible to design each module to optimize efficiency over a wide operating range, including pass-through modes that exhibit very low loss. A 10-kW boost composite converter is experimentally demonstrated having 98.7% efficiency at a critical partial power point, with similar very high efficiencies achieved over a wide operating range.

Resonant Pulse-Shaping Power Supply for Radar Transmitters

Abstract: Typical radar transmitter signals are frequency-modulated with pulsed constant-amplitude envelopes in order to optimize radio-frequency power amplifier (RFPA) efficiency, which results in spectral broadening and power radiated outside of the radar frequency band. This problem can be alleviated by using an appropriately shaped pulse envelope, provided that high-efficiency operation of the radar transmitter is maintained. This paper introduces a pulse-shaping power supply for RFPAs in radar transmitters which enables high efficiency while reducing the spectral emissions. The pulse-shaping power supply is a simple switched resonant circuit capable of approximating a Gaussian pulse envelope waveform. Operating principles are presented and a state-plane-based design approach is described for the resonant pulse-shaping power supply, which enables improved waveform quality and efficiency. An experimental prototype with efficiency greater than 90% is used to supply a 2.14-GHz GaN RFPA. The RFPA efficiency of up to 76%, overall transmitter efficiency of up to 67%, and output signal having high spectral purity demonstrate feasibility of a high-efficiency, high-performance radar system.

Discrete time modeling of output disturbances in the dual active bridge converter

Abstract: The small-signal modeling of dual active bridge (DAB) converter dynamics in discrete time is a useful analysis tool due to the ability to incorporate behavior during zero-voltage switching (ZVS) intervals explicitly, and is advantageous for the direct design of digital compensators. While the control-to-output transfer function has been modeled previously, accurate models of the output impedance of the DAB converter which account for the converter behavior during switching intervals have not been proposed. A discrete time model of the DAB output impedance is developed and tested against experimental results for a 1 MHz, 50-to-4 V, 10 W DAB converter. The model is then extended to the analysis of load current step changes asynchronous to the discrete time sampling instances.

Microfabricated transmission-line transformers with DC isolation

Abstract: A transmission line transformer (TLT) is a device that transforms a circuit impedance and is implemented with interconnected transmission lines. While traditional TLTs, operating at UHF and low microwave frequencies, are constructed from pairs of coaxial lines wound around ferrite cores, various compact implementations at higher frequency without magnetic materials have been demonstrated with multilayer circuit boards, monolithic microwave integrated circuits (MMIC) and air-filled microcoaxial lines implemented in the PolyStrata® wafer-scale technology. Microfabricated Guanella TLTs in the microwave range typically span several GHz of bandwidth, but they are not suitable for impedance matching of power amplifiers and active devices because the input and output ports are shorted to ground and therefore lack DC isolation. A compact and DC-isolated TLT would be especially useful in small-size, low-loss and high-speed DC-to-DC converters.

Digital control of a high-voltage (2.5 kV) bidirectional DC-DC converter for driving a dielectric electro active polymer (DEAP) based capacitive actuator

Abstract: This paper presents a digital control technique to achieve valley switching in a bidirectional flyback converter used to drive a dielectric electro active polymer based incremental actuator. The incremental actuator consists of three electrically isolated, mechanically connected capacitive actuators. The incremental actuator requires three high voltage (~2.5 kV) bidirectional DC-DC converters, to accomplish the incremental motion by charging and discharging the capacitive actuators. The bidirectional flyback converter employs a digital controller to improve efficiency and charge/discharge speed using the valley switching technique during both charge and discharge processes, without the need to sense signals on the output high-voltage side. Experimental results verifying the bidirectional operation of a single high voltage flyback converter are presented, using a film capacitor as the load. Energy efficiency measurements are provided.

Modeling and simulation of conventionally wired photovoltaic systems based on differential power processing SubMIC-enhanced PV modules

Abstract: This paper describes a photovoltaic (PV) power system architecture based on standard wiring of series-connected PV modules, where each PV module includes differential power processing (DPP) submodule integrated converters (subMICs). Given the absence of additional wiring commonly used to allow DPP subMICs to exchange power among PV modules, mismatches in such conventionally wired subMIC-enhanced system may result in bypassed sections, which yields a model with discontinuous-hard-nonlinearities and complicates numerical simulations. The paper presents a simple and efficient solver for the conventionally wired subMIC-enhanced system. The approach is used to examine the mismatch mitigation performance of this architecture in selected utility-scale and residential systems. Although the mismatch mitigation performance is inferior compared to the fully wired DPP subMIC-enhanced system, it is shown that there are cases where the conventionally wired DPP systems offer some energy yield and hot-spot mitigation improvements. Energy yield improvements are more significant in partially shaded systems with multiple parallel strings of modules, and in systems affected by nonuniform aging.

A CMOS controller for submodule integrated converters in photovoltaic systems

Abstract: Substring level differential power processing (DPP) in photovoltaic (PV) power systems is used to mitigate power loss due to mismatches in PV strings. This paper presents the design and implementation of a controller chip in a 0.18 μm CMOS process for a submodule integrated converter (subMIC) in the isolated-port DPP architecture. The presented controller design combines linear and non-linear PWM control for the subMIC MOSFETs while requiring only a few external components. Other features such as power limiting and system disable are included to improve converter and system efficiency. The controller opens the possibilities to minimize the converter footprint, reduce power consumption, and potentially reduce the cost. Experimental results are presented for a 72-cell PV module with three subMICs using the controller chip, demonstrating greater than 99% module-level efficiency at up to 50% mismatch.

Second-order sliding-mode controller for higher-order DC-DC converters

Abstract: The paper adapts the second-order sliding mode (SOSM) control technique to higher-order dc-dc converters. In particular, the paper presents an adaptation of this control technique for buck converters with input filter. The approach consists of regulating the converter output voltage by means of a state-machine similar to the basic SOSM controller, which is shown to perform well as long as the input filter is properly damped, thus ensuring the overall converter stability. Stability and fast dynamic responses are verified by simulations on a 5 V-to-1.3 V point-of-load buck converter with a damped LC input filter.

Design of a high voltage bidirectional DC-DC converter for driving capacitive incremental actuators usable in electric vehicles (EVs)

Abstract: This paper presents the design of a low input (24 V) and variable high output voltage (0–2.5 kV) bidirectional dc-dc converter for driving a capacitive actuator. The topology is a digitally controlled bidirectional flyback converter with a variable frequency control. The objective is, to design the converter for efficiently charging and discharging the capacitive actuator from 0 V to 2.5 kV and vice versa, respectively. The converter is used to drive a dielectric electro active polymer (DEAP) based capacitive incremental actuator, which has the potential to be used in automotive (e.g., EVs), space and medical industries. The design of the bidirectional flyback converter to charge and discharge a 400 nF capacitive actuator is presented, when 4 kV and 4.5 kV high voltage MOSFETs are used on the secondary high voltage side. The experimental results and efficiency measurements of the converter with the proposed design are provided.

Second order sliding mode control of a buck converter with output capacitor ESR and ESL

Abstract: A second order sliding mode (SOSM) buck controller with near-optimal transient response characteristics has previously been described. This paper shows that the performance of this controller can degrade very significantly in the presence of output capacitor equivalent series resistance and inductance (ESR and ESL, respectively). The reasons for this degradation are developed here, and include the introduction of trajectory discontinuities by the ESR/ESL. This paper then presents a modified SOSM buck controller specifically designed to work well in the presence of ESR/ESL. Simulation results demonstrate the superior performance of the new design in a practical point-of-load buck converter.

A framework to share courses among universities: The case of a course on power electronics for electric vehicles

Abstract: The paper describes the organizational framework and contents of a newly developed course on power electronics for electric drive vehicles. The course is developed and taught synchronously among three universities, with each institution individually managing student registration and assessments, and course administration. The participation of instructors and students from different institutions increases the impact of the course. In addition to the regular classes, followed on campus and remote, the high quality material generated by the instructors is available for the students, including a repository of recorded video lecturers and conferences given by specialist in key topics. Interaction with instructors and among students is promoted using a collaborative on-line tool.

Monolithic implementation of phase shifted switched capacitor step-down DC-DC converter for portable power applications

Abstract: This paper presents analysis and design of a reconfigurable switched capacitor (SC) step-down DC-DC converter architecture suitable for on-chip integrated portable power supplies. Comprising basic SC cells, the proposed DC-DC converter is modular in architecture, and can be viewed as a dual of the multiphase buck converter architecture. The SC converter with integrated capacitors is designed in a standard 180 nm CMOS process to operate in 1/3 and 1/2 conversion ratio modes. The converter is designed to operate with input voltage varying from 2.7V to 5.5V, and switching frequency varying from 10 MHz to 70 MHz, while delivering up to 250mA load current at 1.3V output voltage. SPICE simulation results based on detailed process models are presented. It is shown that efficiency is significantly higher than in a linear regulator operating over the complete specified input voltage range, and also reduces the off-chip input and output filter capacitor requirement.